Magneto-rheological fluids undergo a change in apparent viscosity in the presence of a magnetic field. Conventionally, metallurgical powder composition exhibiting magneto-rheological properties are composed of magnetic particles, such as for example ferromagnetic or paramagnetic particles, suspended in a carrier media.
When magneto-rheological compositions are exposed to a magnetic field the magnetic particles of the magneto-rheological composition become polarized and are thereby organized into chains of particles. The chains of particles align to increase the apparent viscosity, or flow resistance, of the overall fluid. In the absence of a magnetic field, the particles return to an unorganized, or free state, and the apparent viscosity, or flow resistance, of the overall material is correspondingly reduced.
Conventional magneto-rheological compositions are described in U.S. Pat. No. 2,667,237 (the 237 patent). The 237 patent teaches a dispersion of paramagnetic or ferromagnetic particles in a liquid, coolant, or semi-solid grease, for example iron powder and light machine oil. In one embodiment the 237 patent describes carbonyl iron powder.
Magneto-rheological compositions are used in linear and rotating mechanisms as a rigid bonding material, such as for example, in braking systems, vehicle suspension dampeners, and power generation devices. In dampening devices, magneto-rheological compositions permit the viscosity of a dampening fluid to be changed in response to an applied magnetic field. Ride stiffness may thereby be controlled by adjusting the current in an electric coil within a dampener. As a result, the stiffness of a suspension system is easily controlled.
The bonding strength of magneto-rheological compositions in the presence of magnetic field depends in part on the strength of the magnetic field applied to the fluid and the size of the magnetic particles. Magneto-rheological compositions having large magnetic particles exhibit a higher yield strength and greater bonding capacity.
Unfortunately, magneto-rheological compositions often suffer performance inconsistency due to the large difference between the specific gravity of the magnetic particles and that of the carrier fluid. As a result, large-sized particles tend to settle out of suspension. For example, U.S. Pat. No. 5,645,752 teaches a magneto-rheological fluid having a thixotropic network to stabilize the particles and prevent settling. Magneto-rheological compositions having smaller sized magnetic particles do not readily settle out of a suspension but exhibit lower yield strength and lower bonding capacity while also having a tendency to “cake up” more easily thereby affecting the fluidity of the composition.
Conventional magneto-rheological compositions also suffer performance degradation over time due in part to oxidation of the magnetic particles, especially in high temperature applications. Therefore, manufacturers continually seek magneto-rheological compositions that resist performance degradation and maintain high yield strength and bonding strength. Hence compositions that satisfy these requirements is desired.